A typical vehicle braking system includes a master cylinder with a power booster that intensifies a manual input force and applies it to the master cylinder to effect pressurization of an associated braking system. Within the master cylinder selective movement of primary and secondary pistons develops elevated fluid pressure which is transmitted to the braking system. During base brake operation, the primary and secondary pistons generate operating fluid pressure which is used to actuate brake calipers or wheel cylinders at each vehicle wheel brake.
Electro-hydraulic brake apply systems are also known wherein the pressure applied to a vehicle's wheel brakes is controlled by a electronic unit that evaluates several parameters and delivers a control signal to a hydraulic modulator that sets the wheel brake pressure. A key parameter used to determine the appropriate braking pressure at the wheel brake is the driver's command, delivered as an input on the brake pedal. Generally, a pressure sensor or brake pedal force sensor monitors apply action providing feedback to the control system for use in setting the braking pressure.
A typical hydraulic modulator includes a pressure generation mechanism and a means of controlling delivery of the generated pressure to the wheel brakes. This may take the form of a pump and proportional hydraulic valve, a pump with a pair of two way valves or a movable piston variable pressure chamber device. The number and arrangement of these elements included in a braking system is determined by the system layout and selected control scheme. With a movable piston variable chamber device, a piston is driven linearly in a bore to vary the size of a pressure chamber. The pressure chamber is connected to a brake line leading to the wheel brake. For an application of braking pressure, the size of the variable chamber is reduced to take up compliance in the system, and to increase braking pressure. The piston applies an increased force to the contained non-compressible fluid to apply the brake. To decrease braking pressure, the force on the piston is reduced and when appropriate, the size of the variable chamber is increased to release the brake.
An automotive braking system may operate without a booster under electrical or electro-hydraulic control in the traditional base brake mode wherein manual actuation of the master cylinder effects a desired application of the wheel brakes with assistance from a remote pressure modulator. In addition to the base brake mode of operation, braking systems are often capable of controlling vehicle deceleration through anti-lock operation, controlling vehicle acceleration through traction control operation, and improving lateral and longitudinal vehicle stability through stability enhancement systems which provide a level of dynamic handling augmentation. During operation of a braking system in a brake-by-wire type of control, the typical master cylinder is isolated from the remainder of the braking system and power is effected through an ancillary pressure generation mechanism such as a motor driven pump or pressure chamber/piston arrangement. In order to effect a fast response time, a relatively large motor and pump combination or a large piston is typically required. Another known method of providing a fast cycle response time is to utilize a separate high pressure accumulator to store a fluid pre-charge, which can be applied to the vehicle wheel brakes when required. These approaches are somewhat undesirable since they tend to increase the overall costs of the system.